Olefin polymerizations such as ethylene polymerization are frequently carried out using monomer, diluent and optional catalyst and optionally co-monomers in a loop reactor. The polymerization is usually performed under slurry conditions, wherein the product consists usually of solid particles and is in suspension of a diluent. The slurry contents of the reactor are circulated continuously with a pump to maintain efficient suspension of the polymer solid particles in the liquid diluent, the product being often taken off by means of settling legs which operate usually on a batch principle to recover the product. Settling in the legs is used to increase the solids concentration of the slurry finally recovered as product slurry. The product is further either transferred to another reactor or discharged to a flash tank, through flash lines, where most of the diluent and unreacted monomers are flashed off and recycled. The polymer particles are dried, additives can be added and finally the polymer is extruded and pelletized.
In these polymerization processes, settling legs, however, do present some problems. They represent the imposition of a “batch” or “discontinuous” technique onto a basic continuous process. Each time a settling leg reaches the stage where it “discharges” or “fires” accumulated polymer slurry it causes interferences on the pressure in the loop reactor, which is thereby not kept constant. Pressure fluctuations in the loop reactor may be larger than 1 bar. At very high monomer concentration, such pressure fluctuations may generate several problems such as the creation of gas bubbles that may cause trouble in the operation of the circulation pump. They may also provoke perturbations in the control scheme of the reactor pressure.
Various alternative product removal techniques are however known. For example, WO 01/05842 describes an apparatus for removing concentrated slurry from a flowing stream of slurry in a conduit characterized by a channel in an outlet area of the conduit, the outlet being adapted to continuously remove slurry.
EP 0891990 describes an olefin polymerization process wherein the product slurry is recovered by means of a continuous product take off, more in particular by means of an elongated hollow appendage provided on the reactor. Said hollow appendage is in direct fluid communication with a heated flash line, and thus being adapted for continuous removal of product slurry.
However the above-described apparatus and processes have the disadvantage that the suspension withdrawn from the reactor still contain a large amount of diluent and of other reactants, such as the monomer, which it is then necessary to subsequently separate from the polymer particles and to treat for the purpose of reusing it in the reactor. Another disadvantage of the above-described apparatus and processes is their lack of flexibility during the phase or reaction start-up or in response to large disruptions in the normal behavior of the reactor, like sudden interruption of one of the feed streams.
WO2011/095532 discloses a process in which settling legs are used for continuous and periodical discharge from a single loop reactor. This document also discloses a process in which two loop reactors are connected in series via settling legs of the first reactor.
It is therefore an object of the present invention to provide a polymerization process occurring in a loop reactor wherein the polymer slurry is efficiently and continuously discharged from the loop reactor and transferred to a subsequent loop reactor. More in particular, it is an object of the invention to optimize the residence time in the settling legs. Another object of the present invention is to improve efficiency of separation of olefin polymer, reactants and diluent. It is further an object of the present invention to improve polymerization process' operability and reliability.